1. Field of the Invention
The present invention relates to a ball joint used for the steering mechanism or the suspension mechanism of an automobile and, more particularly, to a ball joint used at the rack-shaft end of the tie-rod of a rack-and-pinion type steering device.
2. Description of the Prior Art
As shown in FIG. 9, a conventional ball joint includes a housing 1 having an inner chamber 2 containing a hard plastic bearing seat 4 at a bottom 3 thereof. A ball head portion 6 of a ball stud 5 is slidably engaged in bearing seat 4. A projecting stud portion 7, which is integrally connected to ball head portion 6, extends through an aperture 8 of inner chamber 2. A plug 9, fitted into aperture 8, holds bearing seat 4 and ball head 6 of ball stud 5 in inner chamber 2 of housing 1.
The above prior-art structure presents a problem in that it is difficult to absorb dimensional tolerances of the parts that comprise housing 1, bearing seat 4, ball stud 5 and plug 9. As a result, it is difficult to stabilize the characteristics of a ball joint when it is assembled with a tie-rod or other mechanical part. The difficulty in controlling tolerances carries with it the danger that wear of bearing seat 4 and rattling due to setting of bearing seat 4 caused by load, may occur with use.
One approach to solving the above problems is disclosed in Japanese Utility Model Publication No. 7848/1989. As shown in FIG. 10, a ball joint according to the above publication includes an elastic member 14, of synthetic resin, fitted in the bottom 13 of an inner chamber 12 of a housing 11. A bearing seat 15, of hard synthetic resin, is fitted in inner chamber 12. A ball head portion 17 of a ball stud 16 is slidably enveloped in bearing seat 15. A stud portion 18, integrally connected to ball head portion 17, projects from an aperture 19 of inner chamber 12. Aperture 19 of housing 11 is caulked or crimped to hold ball head portion 17 in place, and thereby prevent it from slipping out of inner chamber 12 of housing 11.
According to the structure of a ball joint shown in FIG. 10, it is possible for elastic member 14 to absorb the dimensional tolerances of the principal parts consisting of housing 11, bearing seat 15 and ball stud 16, thereby stabilizing working torque. When load, especially compression load, is applied to ball stud 16, however, the load is conveyed to elastic member 14 through bearing seat 15. If the load is large, it causes setting of elastic member 14, thereby reducing durability.